My Math Forum What's wrong with gravity?

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 November 7th, 2017, 02:53 PM #1 Senior Member     Joined: Nov 2015 From: United States of America Posts: 188 Thanks: 25 Math Focus: Calculus and Physics What's wrong with gravity? Hey forum, During my modern physics lab last night we were working with a simulation that allowed us to visualize wave packets while investigating the probability distribution. We also compared how in classical physics you can't escape the potential well, but in quantum mechanics particles can escape potential wells via tunneling (which is absolutely insane in my opinion!) Anyway, we were discussing if quantum physics and classical physics would ever be fully unified. He said General Relativity does a pretty good job at explaining everything, it's just gravity doesn't fit into the big picture. I asked why gravity doesn't fit in, and he said that takes years of classes to understand... So I'm curious, what's up with gravity? Does it just not fit into quantum mechanics? I thought general relativity was a theory of gravitation so shouldn't that explain it? (Question was framed ignorantly intentionally, I am curious!) Thanks!
 November 7th, 2017, 04:18 PM #2 Senior Member     Joined: Sep 2015 From: USA Posts: 2,317 Thanks: 1230 take a look at this General Relativity is a classical theory in that everything operates on the continuum. The video above talks about why conventional methods of quantizing field theories, which were very successful for say QED, don't work for gravity. Thanks from topsquark and SenatorArmstrong
November 7th, 2017, 07:42 PM   #3
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 Originally Posted by SenatorArmstrong Hey forum, During my modern physics lab last night we were working with a simulation that allowed us to visualize wave packets while investigating the probability distribution. We also compared how in classical physics you can't escape the potential well, but in quantum mechanics particles can escape potential wells via tunneling (which is absolutely insane in my opinion!) Anyway, we were discussing if quantum physics and classical physics would ever be fully unified. He said General Relativity does a pretty good job at explaining everything, it's just gravity doesn't fit into the big picture. I asked why gravity doesn't fit in, and he said that takes years of classes to understand... So I'm curious, what's up with gravity? Does it just not fit into quantum mechanics? I thought general relativity was a theory of gravitation so shouldn't that explain it? (Question was framed ignorantly intentionally, I am curious!) Thanks!
This turned into a book. Sorry about that!

I'd like to throw my two cents in here. (Maybe I should use quarters to keep up with inflation?)

Anyway, I didn't watch the video to see if it is mentioned... The Hamilton-Jacobi equation, which gives a slightly different description of Classical Mechanics than, say Lagrangian Mechanics, can be used to bridge the gap between Classical and Quantum. The Mathematics are as nasty as anything I've ever seen but it does work better than any other methods I've seen. And yes, tunneling is weird but it's been measured in x-ray diffraction experiments. Welcome to the rabbit hole of Quantum Physics!

As far as GR is concerned there are a large number of Physicists and Mathematicians who are working on how to bring gravity into the fold of QM, but as the video is sure to say, there are problems. On the other hand gravity doesn't really have that much of an effect on the Quantum scale except in situations where the gravitational fields are rather large. This doesn't happen very often except when we talk about very small very massive particles such as a Quantum black hole.

And IMHO we should keep working on Quantum Gravity but it is quite possible that we may never have one because there's nothing to say that it has to be. It's a neatness thing...We want to have all four (known) forces to be manifestations of a single "super-force" which is slightly easier to visualize than having four distinct forces that have nothing to do with each other. This viewpoint is, of course, somewhat heretical.

-Dan

 November 7th, 2017, 09:37 PM #4 Senior Member   Joined: May 2016 From: USA Posts: 1,306 Thanks: 549 I am not a physicist so I am not competent to make a diagnosis of what is wrong with gravity. But the symptoms indicate that it has been behaving poorly since at least the 16th century. First, it was ellipses instead of circles. Then it was gravity's perverse refusal to act linearly. Most recently, gravity has refused to be sociable with the other fundamental forces. Thanks from topsquark, Country Boy, SenatorArmstrong and 1 others

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